1. Field of Invention
This invention relates to a Coriolis type mass flowmeter; and more particularly, to such flowmeter wherein high speed calculations are performed without any substantial amount of calculation error.
2. Description of the Prior Art
A conventional Coriolis type mass flowmeter is depicted, for example, in FIGS. 1, 2 and 3, wherein FIG. 1 shows the construction of a sensor unit of a conventional Coriolis type mass flowmeter, FIG. 2 shows the operation of the sensor unit of FIG. 1, and FIG. 3 shows a converter for calculating mass flow in combination with the sensor unit of FIG. 1. In the specification, a straight tube is used as the measuring tube; however, other types of tubes may be used, such as, for example, a U-type tube, etc.
FIG. 1 shows a measuring tube 1 through which fluid being examined (also called "measurement target") flows. Both ends of tube 1 are fixed to support members 2 and 3. An exciter 4, for mechanically exciting , i.e. vibrating, tube 1 in a vertical direction is disposed near the center of tube 1. Sensors 5A and 5B are provided for detecting the vibration of tube 1 and are positioned near parts of tube 1 fixed to support members 2 and 3. A temperature sensor 6, which is used for temperature compensation, is disposed near support member 3. The foregoing components considered together may constitute sensor unit SNS.
When the fluid being examined flows through tube 1, while vibration in a primary mode represented by M1 and M2 in FIG. 2, is applied by exciter 4 to tube 1, tube 1 is vibrated in a secondary mode represented by M3 and M4 in FIG. 2. In practice, tube 1 is vibrated in such a vibration mode or pattern that the M1,M2 and M3,M4 types of vibration patterns are superposed on each other. The deformation of tube 1, due to vibration, is detected by sensors 5A,5B. The detection results are transmitted as displacement signals S.sub.A and S.sub.B to a converter TR1 as shown in FIG. 3.
Displacement signal S.sub.A, which is detected by sensor 5A, is inputted to a frequency measuring circuit 7 to measure the signal frequency f.sub.A of displacement signal S.sub.A. The signal frequency f.sub.A is outputted to a timing generator 8 to generate a timing signal T.sub.S which is used for sampling. Timing signal T.sub.S is used to indicate the time points of N which are obtained by accurately equally dividing one vibration period of tube 1 into equal number of parts N, wherein N represents an integer.
Also, displacement signal S.sub.A is outputted to a track and hold (T & H) circuit 9, whereat the signal s.sub.A is successively sampled and held at each of the N time points of each period of displacement signal S.sub.A with the sampling timing signal T.sub.S produced by timing generator 8. The held displacement signal S.sub.A is outputted to an analog to digital (A/D) converter 10 to be successively converted to a digital signal D.sub.A1.
Digital signal D.sub.A1 is subjected to a Fourier transformation to be converted to a frequency area in a discrete Fourier transform (DFT) circuit 11, whereat a phase .theta..sub.A1 is calculated on the basis of the ratio of a real number component and an imaginary number component of the converted signal.
On the other hand, displacement signal S.sub.B, which is detected by sensor 5B, is outputted to a track and hold (T & H) circuit 12, whereat signal S.sub.B is successively sampled and held at each of the N time points of each period of displacement signal using sampling time signal T.sub.S from timing generator 8. The held displacement signal S.sub.B is outputted to an analog to digital (A/D) converter 13 whereat the signal is successively converted to a digital signal D.sub.B1.
The digital signal D.sub.B1 is subjected to Fourier transformation to be converted to a frequency area in a discrete Fourier transform (DFT) circuit 14, whereat a phase .theta..sub.B1 is calculated on the basis of the ratio of a real number component and an imaginary number component of the converted signal.
A phase difference calculation circuit 15 receives signals having phases .theta..sub.A1 and .theta..sub.B1 from DFT circuits 11 and 14 and then calculates the difference between such signal phases, and then successively outputs the calculation results as a phase difference signal .theta..sub.1, that is: EQU .theta.=.theta..sub.A1 -.theta..sub.B1 =.PHI. (1)
Next, tan .PHI. is calculated on the basis of the phase difference signal .theta..sub.1 (=.PHI.), and then divided by signal frequency f.sub.A to calculate the mass flow. At the same time, tan .PHI. is subjected to temperature compensation using a temperature signal detected by sensor 6 and in circuit (not shown) to thereby produce an accurate mass signal.
Displacement signal S.sub.A is applied to an exciting circuit 16, and an exciting voltage, corresponding to the displacement signal, is outputted to the exciter 4 (see FIG. 1) to drive exciter 4 in a sine wave form, for example.
However, the conventional Coriolis type mass flowmeter, such as described above, needs a timing generator 8 becauses it uses sample values at the N points which are obtained by accurately dividing equally one period of the vibration frequency of the measuring tube into N equal parts. In addition, the vibration frequency varies in accordance with density, temperature, etc., of the fluid being examined, so that the measurement value also varies. Thus, measurement becomes unstable. Accordingly, the conventional Coriolis type mass flowmeter has a disadvantage in that the output cannot follow the variation with any high degree of precision.
Moreover, in the flowmeter, tan .PHI. is calculated on the basis of the phase difference signal .theta..sub.1, so that the speed of calculation is slow, and errors of calculation tend to occur.